In recent years, the amount of information to be recorded in a single optical recording medium has been increasing, and the distribution of information recorded in optical recording media as software information has been promoted. The technology to prevent illegal copying thereof also has developed. Therefore, as so-called security measures, identification information is desired to be recorded respectively in optical recording media.
To meet this requirement, the technique generally applied to record identification information for optical recording media is, for example, to provide a pit part of a reproduction-only type optical recording medium with an additional area (Burst Cutting Area, hereinafter abbreviated as “BCA”) where bar-codes are overwritten, so that identification information (ID) and, if necessary, an encryption key or a decryption key can be recorded in the BCA during the manufacturing process of optical recording media.
One example of recording signals in this BCA of a reproduction-only type optical recording medium is illustrated in FIG. 14. Specifically, as shown in FIG. 14(1), based on signals modulated according to predetermined identification information such as ID, a pulse-like laser beam is emitted in accordance with a pattern of the BCA so as to break and remove the reflection film of the optical recording medium at a stretch in a stripe form as illustrated in FIG. 14(2). The stripe-form BCA is formed on the optical recording medium with portions where the reflection film is broken and removed and with portions where the reflection film remains, as shown in FIG. 14(3). When this stripe-form BCA pattern is reproduced by an optical head of an optical information recording/reproduction device, the modulated signals show a waveform in which some portions are missing intermittently, since the reflection film has been removed in the BCA part. These missing portions in the waveform then undergo a filter process as shown in FIG. 14(5), and the digital reproduction data are detected as shown in FIG. 14(6). In this way, the identification information recorded on the optical recording medium can be obtained. By reading this identification information, each optical recording medium can be specified.
On the other hand, optical recording media are getting diversified more and more, and not only recording-type optical information recording media provided with an information layer capable of recording information signals have been developed, but also rewritable-type optical information recording media provided with an information layer capable of rewriting information signals freely. Since information can be recorded freely in this recording-type optical information recording medium as well as in this rewritable-type optical information recording medium (hereinafter referred to as an “optical disc” for both recording- and rewritable-types), it has become increasingly important to take the security aspect of information recorded in optical discs into consideration.
However, when the method for forming a BCA pattern by breaking and removing a reflective layer in a reproduction-only type optical recording medium is to be applied to an optical disc, the following problems arise.
First, in the information layer containing one of photoactive materials selected from pigment, magnetic materials and phase change type recording materials, the presence of information is detected by an optical change of the information layer itself. Therefore, even if the optical disc is provided with the reflective layer, an optical difference hardly can be detected by the BCA pattern in which only the reflective layer is removed. Thus, it is necessary to generate an optically detectable change of the information layer itself that can record information signals in optical discs.
Next, even if the method for forming a BCA pattern in a reproduction-only type optical recording medium is applied by emitting a laser beam pulse-like in accordance with the BCA pattern so as to break and remove the information layer in the optical disc, it is not possible to selectively break and remove only the information layer containing a photoactive material, since a laminated film such as an enhancement layer, a hard layer, an intermediate layer, or a dielectric layer is formed at least on one side of the information layer. As a result, the information layer and/or the laminated film in the vicinity of the boundary of the BCA pattern may be separated, or particle of the information layer and/or the laminated film arising inside the BCA pattern may cause distortion in forming the BCA pattern part, which leads to the problem that BCA signals cannot be obtained sufficiently due to noise mixed in the signals for detecting the BCA.
Furthermore, the defect caused by the separation of the information layer and/or the laminated film in the vicinity of the BCA pattern is not confined to the subsidiary information area but reaches the information layer and/or the laminated film of the main information area, which is a critical problem for the recording-type optical recording medium.
In particular, in the phase change type optical disc, after a light beam with a pulse modulated according to the information signal is emitted to the information layer to melt the information layer, the information layer is cooled, and thus, a recording mark is formed, so that information is recorded. Since the process of melting the information layer is included herein, for the purpose of suppressing the phenomenon of the pulsation or the float of the photoactive material of the information layer in the molten state, which causes changes in the recording characteristics, the configuration of providing the information layer with a material generally referred to as a dielectric, which has better thermomechanical characteristics than the materials comprising the information layer, is adopted in which the dielectric is present by contacting the information layer. Furthermore, the rewritable type optical disc in which the phase state changes reversibly has the configuration of holding the information layer between dielectrics.
The laminated film having the function of suppressing the phenomenon such as the pulsation and/or the float of the photoactive material in this information layer at the time of melting acts to hinder the formation of a BCA pattern when the BCA pattern is formed. If high energy is emitted forcibly to form a BCA pattern, due to the lack of the place for absorbing the impact such as boiling or evaporation of the photoactive material, the laminated film and/or the information layer may be separated, or bubbles, depressions and particles of the material of the information layer and/or the laminated film may be created inside the BCA pattern and in the peripheral portion thereof. Accordingly, the defect spreads to the subsidiary information area and also to the information layer in the main information area, thereby increasing the factors of causing critical defects that make it impossible to record.
Thus, it is difficult to record BCA patterns that can be detected exactly at least in the recordable type optical disc. One of the main reasons for the increase in the manufacturing costs of optical discs is the problem related to the formation of BCA patterns.